It is well known to use SAW devices for filtering purposes in microwave radio transmission systems. With increasing capacities of such systems, very stringent demands have been placed on the design and fabrication of such SAW devices, so that they have been developed to an advanced stage. For example, Suthers et al. U.S. Pat. No. 4,814,658 issued Mar. 21, 1989 and entitled "SAW DEVICE WITH CONTINUOUS FINGER PATTERN" describes and claims an advanced form of SAW device which is particularly useful for filtering purposes in microwave radio systems, and discusses earlier developments embodied in such a SAW device.
In addition to filters, microwave radio systems include other circuits, such as equalizers, which can benefit by the incorporation therein of appropriate SAW devices. In particular, a microwave radio receiver invariably includes an ATDE (automatic time domain equalizer), which may incorporate a transversal filter comprising a tapped delay line.
In conventional receivers of microwave radio systems which use QAM (quadrature amplitude modulation, in which signals are modulated onto two carrier signal components, referred to by the letters I and Q, in phase quadrature), the ATDE has typically been provided at baseband, relatively separately for the I and Q components. This has necessitated duplication of many parts of the ATDE for the I and Q components. This is a particular disadvantage for ATDEs with delay lines with a large number of taps, for which there is an increasing requirement with increasing capacity of the radio system.
In order to avoid this disadvantage, it is possible to implement the ATDE in the IF (intermediate frequency, typically 70 MHz) stages of the radio receiver. In such an IF ATDE, the tapped delay line has to date had to be constituted by a series of lumped delay elements, which are bulky and expensive and have to be tuned by skilled persons. These factors become increasingly disadvantageous as the number of taps of the delay line increases.
It is desirable, therefore, to replace the series of lumped delay elements by a tapped delay line constituted by a SAW device, which is suited to operation at the intermediate frequency. However, the nature and properties of SAW devices, some of which are discussed further below, make it very difficult to provide a SAW device which can meet the stringent demands imposed by a high capacity microwave radio system.
In an article by Carl M Panasik et al. entitled "A 32 TAP DIGITALLY CONTROLLED PROGRAMMABLE TRANSVERSAL FILTER", 1988 Ultrasonics Symposium, pages 151 to 154, there is described a digitally controlled programmable transversal filter (DCPTF) which uses a SAW device providing a delay line with 32 taps. This DCPTF is intended for use as a bandpass filter with a programmable center frequency and bandwidth. A radio frequency signal is supplied to an input IDT (inter-digital transducer) of the SAW device, and the resulting SAW propagated towards 32 sequentially spaced output IDTs results in variously delayed "copies" of the input signal at the output taps. These are weighted by respective tap weight control amplifiers, each constituted by a segmented dual-gate FET pair with binarily scaled gains, duplicated for positive and negative polarities, whose outputs are summed to produce a programmably filtered signal.
Although this article discloses a SAW device tapped delay line, such a device is unsuitable for use in a microwave radio receiver IF ATDE as discussed above in view of the stringent demands thereof. There are a number of reasons for this, arising from the nature and properties of SAW devices. Apart from considerations of reflections which are considered in the Suthers et al. patent referred to above, particular factors of importance in relation to a SAW device tapped delay line relate to acoustic regeneration at the output IDTs and reflections among these IDTs.
Acoustic regeneration at each IDT occurs in that an incident SAW generates a voltage across the IDT, which in turn regenerates SAWs which propagate from the IDT in both directions, these regenerated SAWs being incident on adjacent IDTs. Reflections among IDTs arise in that mass and electrical loading (MEL) imposed by the IDT fingers on the substrate of the SAW device produce impedance discontinuities, and hence SAW reflections, at the finger edges. Within the individual IDTs these reflections can be made self-cancelling by the use of split finger pairs as is known in the prior art, but this is not possible in an IF ATDE SAW device for reflections between the output IDTs. This is because the finger spacing or pitch is determined by the center frequency (i.e. the IF) at which the SAW device is designed to operate, whereas the spacing or pitch of the output IDTs must be equal to the inverse of the symbol rate of the microwave radio system, which is generally unrelated to this center frequency.
An object of this invention, therefore, is to provide an improved SAW device tapped delay line which is particularly suitable for use in an IF ATDE of a microwave radio receiver.